I do see a different cam used. 

335 HP        .398 lift intake .398 lift exhaust   322 duration              3874872

390 HP        .461 lift intake .480 lift exhaust     350/352 duration     3883986

1969 saw the introduction of a third 427 power plant, the LS-1 335 horsepower variant. This engine was a direct replacement of the L-35 325 horsepower 396, which was phased out of the full size Chevrolet lineup after 1968. The LS-1 was NOT an available engine with SS427 equipment, but I have included it because inevitably many of the original LS-1 cars in 1969 end up as clones.

A mild performer, it was a very workman-like engine that offered both power and streetability, and was available in all full-sized Chevrolets. The most popular of the 427s in 1969, the LS-1 was installed in 18,308 passenger Chevrolets. As was stated earlier, the LS-1 was not listed as an available engine for the 1969 SS427. However, it is not outside the realm of possibility that some of these engines found their way into SS427 Impalas in 1969. Anything seemed possible back then, so I included the LS-1 statistics on this page.

Notice in the image above the style of air cleaner lid; this elongated "neck" is correct for the LS-1. Notice the difference between this air cleaner lid and that of the L-36 which used more of a snub nosed style. Often times this can clue you in to whether or not you are looking at a 335 horsepower 427; assuming the original air cleaner is still with the vehicle!

The engine suffix codes pertaining to the 1969 LS-1

I think dual exhaust has something to do with it as well. All Pontiac literature says to add 15% with N10 dual exhausts throughout. 

335 x 1.15 = 385

The LS5 is likely underrated for public consumption to keep it under 400 Gross HP.

The LS1 was used in high torque need situations like a big B Body station wagon. 

Standard dual exhaust on the 390 HP 427.